Legionnaires' disease is a severe form of pneumonia caused by a type of bacteria called Legionella, according to the CDC. This colorized scanning electron micrograph with moderately-high magnification depicts a large grouping of Gram-negative Legionella pneumophila bacteria. Photo Credit: Wikimedia Commons / CDC

New York City has identified 46 cases of Legionnaires' disease in the past three weeks, all clustered in the South Bronx, Mayor Bill de Blasio said Thursday -- an increase from the 31 cases reported a day earlier.

The disease was fatal for a man and woman in their 50s who both had already been sick with other ailments, officials said.

"We're now beginning hospital checks every day," Dr. Mary Bassett, the city's health commissioner, said at a City Hall news conference.

Sources of contamination have been identified in cooling towers at Concourse Plaza -- a commercial complex with a shopping mall and multiplex theater -- and city-owned Lincoln Hospital, de Blasio said.

"At both sites, decontamination is occurring as we speak," he said.

His office said operations at the locations has not been affected. Bassett said none of the infections have been connected to the contaminated cooling towers. Ten other potential disease source sites are being tested, the city said.

Consulting maps and aerial photographs, Bassett's Department of Health and Mental Hygiene is in the process of finding and testing all rooftop water towers and cooling systems across the South Bronx.

Bassett said that the 46 cases aren't necessarily new diagnoses, but some had just come to the attention of city epidemiologists. The patients are being interviewed, and their records scrutinized, to identify common sources of exposure.

Most of the victims are older, and that those at higher risk include smokers, those with lung disease and already-weakened immune system, she said.

The first cases were reported July 10. Most were in the High Bridge-Morrisania and Hunts Point-Mott Haven neighborhoods. Symptoms of the ailment include cough, shortness of breath, high fever, muscle aches and headaches.

The disease -- a type of bacterial pneumonia so named because it was discovered after an outbreak in at an American Legion meeting in 1976 in Philadelphia -- is transmitted through air conditioning, showers, baths, cooling towers or other water sources. It cannot be transmitted between people.